Time-shift viewing means that when a viewer interrupts viewing of a broadcast content (typically, television broadcasting, streaming, and the like), a multicast content, or a unicast content, viewing is restarted using the same device or another device from the point where the viewing is interrupted. Time-shift viewing includes not only restart of the interrupted viewing but also particular replay such as pause or rewinding.
In viewing of these contents, as a viewer often has to interrupt viewing due to a private matter and move to another place, it is required that the viewer can restart viewing easily in such a case, and in particular, restart viewing using a different device for viewing.
Techniques for content distribution systems considering time shift are disclosed in Patent Documents 1 to 3, for example. Patent Document 1 discloses a content distribution system in which when a viewer interrupts viewing of a broadcast content by a first broadcast receiver, if a time-shift replay is instructed from a second broadcast receiver, it is possible to replay the broadcast content stored in the first broadcast receiver from the point where the viewing was interrupted.
Patent Document 2 discloses a content distribution system including a transmitter and a receiver connected over a network, in which when a viewer interrupts viewing by the receiver of a multicast content transmitted from the transmitter, the multicast content can be replayed from the point where the viewing was interrupted then the viewing is restarted.
Patent Document 3 discloses a content distribution system, in which by sharing replay positional information among a plurality of streams in a common content, an authorized log-in user can easily continue viewing of the content regardless of the receiving environment or receiving terminals.    Patent Document 1: Japanese Unexamined Patent Publication 2004-363659    Patent Document 2: Japanese Unexamined Patent Publication 2004-088466    Patent Document 3: Japanese Unexamined Patent Publication 2004-056542